Semiconductor spatial light modulators (SLM's) are one viable solution to realizing high quality, affordable imaging systems such as xerographic printers and displays. One promising SLM technology suitable for both printers and displays is the digital micromirror device (DMD) manufactured by Texas Instruments Incorporated of Dallas, Tex. The DMD is a monolithic semiconductor device having a linear or area array of bi-stable movable micromirrors fabricated over an array of corresponding addressing memory cells. One embodiment of a xerographic printer implementing a tungsten light source focused via optics on an imaging DMD mirror array is disclosed in U.S. Pat. No. 5,041,851 to Nelson, entitled "Spatial Light Modulator Printer and Method of Operation", assigned to the same assignee as the present application and the teachings included herein by reference.
In an imaging system implementing an imaging DMD spatial light modulator, it is desired to uniformly illuminate the DMD mirror array with a homogeneous light source such that each pixel mirror of the array modulates a uniform intensity portion of light. This is necessary in printing because the DMD mirror array modulates this light to expose a light sensitive rotating organic printing drum, whereby the intensity and duration of the modulated light directed thereon determines the relative exposure of the charged drum. In displays, the intensity of the modulated light determines the perceived brightness and colors of the image.
It is also necessary that the energy of the light directed upon the DMD mirror array be of sufficient flux per unit area to maintain the contrast ratio. If insufficient light energy is modulated and directed to the drum by the DMD mirror array, the printing drum may not be fully exposed, thus degrading the contrast of the image printed on a printing medium, or the displayed image.
U.S. Pat. No. 5,159,485 to Nelson, entitled "System and Method for Uniformity of Illumination for Tungsten Light", assigned to the same assignee of the present invention and the teachings included herein by reference, discloses an anamorphic optical path arranged such that the vertical component of the source light is compressed to match the physical shape of the DMD mirror array. The embodiment disclosed dramatically increases the optical efficiency of the system, whereby light energy is compressed to irradiate the DMD mirror array more intensely from a given light source, such as a tungsten lamp.
U.S. Pat. No. 5,151,718 to Nelson, entitled "System and Method for Solid State Illumination for DMD Devices", also assigned to the same assignee of the present invention and the teachings included herein by reference, discloses an array of LED emitters constructed to efficiently replace the conventional tungsten source lamp. The LED array is geometrically configured, and can be electrically operated by strobing to vary the brightness of light to individual mirror pixels to achieve gray scale imaging, and reduce fuzzy line images. Each of the LED's in the array can be provided with a lens to help columnate the light through optics and onto the DMD mirror array. Using LED's , light is efficiently directed and focused onto the DMD mirror array, with little light being wasted and directed elsewhere. Less optical energy is required of the light source compared to a conventional tungsten lamp to illuminate the DMD mirror array with a particular light intensity. The LED's can be quickly turned on and off, thereby providing the ability to modulate the light energy directed upon the DMD mirror array, and consequently, helps achieve gray scale printing. For instance, during a given line print cycle, the LED can be on for 50% of the cycle time to irradiate the DMD array with half the light energy available for that particular time interval. However hi-power arrays of multiple LED emitters are relatively expensive compared to conventional tungsten lamps. Moreover, the best state of the art LED devices suitable for electro-photography are known to only reliably produce 200 milliwatts each. Accordingly, the alignment of the optics is critical to ensure that the energy of each LED is directed upon the DMD mirror array. That is to say, the LED array may not produce sufficient and uniform light energy should one LED fail.
U.S. Pat. No. 5,105,207 to Nelson, entitled "System and Method for Achieving Gray Scale DMD Operation", assigned to the same assignee as the present invention and the teachings incorporated herein by reference, discloses a system for enhancing resolution of a xerographic process by submodulation of each individual pixel. The submodulation is achieved by anamorphically reducing the square pixel presentation of light rays to a rectangle having a number of controllable segments within each square pixel scanned line. A conventional tungsten lamp is incorporated in this embodiment.
It is desirable to provide a low cost, high intensity optical system whereby the DMD mirror array can be uniformly illuminated with high intensity light. Moreover, it is desirable to provide a high intensity light source which can be modulated in intensity to effect gray scale printing. Providing a low cost, single light source is preferred. The optical system should be easy to align, whereby any degradation in the light source would be uniformly presented to the DMD array, and would not noticeably degrade the printing quality of the xerographic printer.